Those nasty weeds - poison ivy, poison sumac, and poison oak - are the single most common cause of allergic reactions in the United States. Each year 10 to 50 million Americans develop an allergic rash after contact with these poisonous plants.

Poison ivy, poison oak, and poison sumac grow almost everywhere in the United States, except Hawaii, Alaska, and some desert areas of Nevada. Poison ivy usually grows east of the Rocky Mountains and in Canada. Poison oak grows in the Western United States, Canada, and Mexico (western poison oak), and in the southeastern states (eastern poison oaks). Poison sumac grows in the eastern states and Southern Canada.

Poison Ivy Rash

Poison ivy rash is an allergic contact rash (dermatitis) caused by contact with an oil called urushiol (you-ROO-shee-ol). Urushiol is found in the sap of poison ivy, poison oak, and poison sumac. It is a colorless or pale yellow oil that oozes from any cut or crushed part of the plant, including the roots, stems, and leaves. After exposure to air, urushiol turns brownish-black, making it easier to spot. Contact with urushiol can occur in three ways:

Direct contact - touching the sap of the toxic plant.

Indirect contact - touching something to which urushiol has spread. The oil can stick to the fur of animals, to garden tools, to sports equipment, or to any object that has come into contact with a crushed or broken plant.

Airborne urushiol particles, such as from burning plants, that may come in contact with your skin.

Once urushiol touches the skin, it begins to penetrate in minutes. In those who are sensitive, a reaction appears as a line or streak of rash, usually within 12 to 48 hours. Redness and swelling occur, often followed by blisters and severe itching. In a few days, the blisters may become crusted and begin to scale. The rash takes 10 days or longer to heal.

The rash can affect almost any part of your body, especially where your skin is thin, such as on your face. A rash develops less often on the soles of your feet and palms of your hands, where the skin is thicker. The rash does not spread, although it may seem to when it breaks out in new areas. This may happen because urushiol absorbs more slowly in to skin that is thicker, such as on your forearms, legs, and trunk.

Who's Sensitive, Who's Not

We are not born with a sensitivity to poison ivy. Sensitivity develops after the first direct skin contact with the oil urushiol. An allergic reaction seldom occurs on the first exposure. A second encounter can produce a reaction, which may be severe. About 85 percent of all people will develop an allergic reaction when adequately exposed to poison ivy.

This sensitivity varies from person to person. People who reach adulthood without becoming sensitive have only a 50 percent chance of developing an allergy to poison ivy. However, do not assume that you are one of the few people who are not sensitive. Only about 15 percent of people seem to be resistant.

Sensitivity to poison ivy tends to decline with age. Children who have reacted to poison ivy will probably find that their sensitivity decreases by half in young adulthood without repeated exposure. People who were once allergic to poison ivy may even lose their sensitivity later in life.

Some people are very sensitive to poison ivy. They can develop a severe rash with blisters and extreme swelling on their face, arms, legs, and genitals. Such severe cases need medical treatment.

Learn to identify the poison ivy plant, and you will have taken the first step in avoiding poison ivy. The popular saying "leaves of three, beware of me," is a good rule of thumb for poison ivy and poison oak but is only partly correct. A more exact saying would be "leaflets of three, beware of me," because each leaf has three leaflets. Poison sumac, however, has a row of paired leaflets. On each of these plants, the middle or end leaflet is on a longer stalk than the other two or more leaflets. This differs from many other look-alikes.

Poison ivy can have different forms. It grows as a vine, climbing vine, or low shrub. Poison oak, with its oak-like leaves, is a low shrub in the east and can be a low or high shrub in the west. Poison sumac grows to a tall shrub or small tree. The plants also differ in where they grow. Poison ivy grows in fertile, well-drained soil. Western poison oak grows wherever there is enough water, and eastern poison oak prefers sandy soil but sometimes grows near lakes. Poison sumac tends to grow in standing water, such as peat bogs.

These weeds are most dangerous in the spring and summer. That's when there is plenty of sap and the plants easily bruise. The leaves may have black marks where they have been injured. Although poison ivy rash is usually a summer complaint, cases sometimes occur in winter, when people burn wood that has urushiol on it or cut poison ivy vines for wreaths.

Know how to recognize these toxic plants in all seasons. In the early fall, the leaves can turn colors such as yellow or red when other plants are still green. The berry-like fruit on the mature female plants also changes color in fall, from green to off-white, and in the winter the plants lose their leaves. In the spring poison ivy has yellow-green flowers.

What To Do About Poison Ivy

Prevent the misery of poison ivy by looking out for the plant when you are outdoors, and staying away from it. You can destroy these weeds with herbicides in your own backyard, but this is not practical elsewhere. If you are going to be where you know poison ivy likely grows, wear long pants and long sleeves, boots, and gloves. Remember that the plant's nearly invisible oil, urushiol, sticks to almost all surfaces. Do not let pets run through wooded areas since they may carry home urushiol on their fur. Because urushiol can travel in the wind if it burns in a fire, do not burn plants that look like poison ivy.

Barrier skin creams such as a lotion containing bentoquatum offer some protection before contact with poison ivy, poison oak, or poison sumac. This over-the-counter product prevents urushiol from penetrating the skin. Ask your dermatologist for details.

Treatment - A Poison Ivy Primer

If you think you've had a brush with poison ivy, poison oak, or poison sumac, follow these simple steps:

Wash all exposed areas with cold running water as soon as you can reach a stream, lake, or garden hose. If you can do this within five minutes, the water may keep the urushiol from contacting your skin and spreading to other parts of your body. Within the first 30 minutes, soap and water are helpful.

Wash your clothing with a garden hose outside or in a washing machine with detergent. If you bring the clothes into your house, be careful that you do not transfer the urushiol to rugs or furniture. You may also dry clean contaminated clothes. Because urushiol can remain active for months, wash camping, sporting, fishing, or hunting gear that was in contact with the oil.

Relieve the itching of mild rashes by taking cool showers and applying over-the-counter preparations like calamine lotion or Burrow's solution. Soaking in a lukewarm bath with an oatmeal or baking soda solution also may ease itching and dry oozing blisters. Over-the-counter hydrocortisone creams are not strong enough to have any effect on poison ivy rashes.

In severe cases, prescription cortisone can halt the reaction if used early. If you know you have been exposed and have developed severe reactions in the past, consult your dermatologist. He or she may prescribe cortisone or other medicines that can prevent blisters from forming. If you receive treatment with a cortisone-like drug, you should take it longer than six days, or the rash may return.

Common Myths About Poison Ivy

Scratching poison ivy blisters will spread the rash.

False. The fluid in the blisters will not spread the rash. Before blisters form, the rash is spread by urushiol on your hands, for instance, by scratching your nose or wiping your forehead. Avoid excessive scratching of your blisters. Your fingernails may carry bacteria that could cause an infection.

Poison ivy rash is "catching."

False. The rash is a reaction to urushiol. The rash cannot pass from person to person; only urushiol can be spread by contact.

Once allergic, always allergic to poison ivy.

False. A person's sensitivity changes over time, even from season to season. People who were sensitive to poison ivy as children may not be allergic as adults.

Dead poison ivy plants are no longer toxic.

False. Urushiol remains active for up to several years. Never handle dead plants that look like poison ivy.

One way to protect against poison ivy is by keeping yourself covered outdoors.

True. However, urushiol can stick to your clothes, which your hands can touch and then spread the oil to uncovered parts of your body. For uncovered areas, barrier creams are sometimes helpful. Learn to recognize poison ivy so you can avoid contact with it.